Silicon carbide based porous material and method for production thereof

ABSTRACT

A silicon carbide-based porous material characterized by comprising silicon carbide particles as an aggregate, metallic silicon and an oxide phase containing Si, Al and an alkaline earth metal; it is high in porosity and strength and superior in oxidation resistance and thermal shock resistance and, when used as a filter, is very low in risk of having defects such as cuts (which cause leakage of fluid) and the like, as well as in pressure loss.

TECHNICAL FIELD

The present invention relates to a silicon carbide-based porous materialsuitable for use as a material constituting mainly a filter forpurification of automobile exhaust gas, a catalyst carrier, or the like,as well as to a method for production thereof.

BACKGROUND ART

A porous honeycomb structure constituted by cell partition walls (ribs)forming an assemblage of a plurality of cells adjacent to each other anda honeycomb outer wall surrounding and holding the outermost cellsconstituting the circumference of the assemblage of cells, are in wideuse as a filter (a diesel particulate filter, i.e. a DPF) for trappingand removing the particulate substance contained in aparticle-containing fluid such as diesel engine exhaust gas or as acatalyst carrier for carrying a catalyst component for purification ofharmful substance in exhaust gas. Also, refractory silicon carbide (SiC)is in use as a material for the above porous honeycomb structure.

Development works are being pushed forward for a DPF (a DPF for catalystregeneration) which comprises a conventional DPF and an oxidationcatalyst carried thereon and which can oxidize and burn the particulatesdeposited on the catalyst to continuously regenerate the catalyst.

As such a honeycomb structure, there is disclosed, for example, a poroussilicon carbide-based catalyst carrier of honeycomb type which isobtained by forming a silicon carbide powder (a raw material) having adesired specific surface area and containing impurities, into a formedmaterial of intended shape, drying the formed material, and firing thedried formed material in a temperature range of 1,600 to 2,200° C. (e.g.Patent Literature 1).

In the case of the catalyst carrier disclosed in Patent Literature 1, inthe sintering (necking) by the recrystallization reaction of the siliconcarbide powder per se, the silicon carbide component vaporizes from thesurface of the silicon carbide particles and condensates at the contactareas (necks) between the silicon carbide particles, whereby the necksgrow and a bonded state results. However, the vaporization of siliconcarbide requires a very high sintering temperature, which has invited ahigh cost; moreover, a material of high thermal expansion coefficientneed be sintered at a high temperature, which has resulted in a lowsintering yield. Further, when it is attempted to produce a filter ofhigh porosity, particularly high porosity of 50% or more by theabove-mentioned sintering by the recrystallization reaction of thesilicon carbide powder per se, the sintering mechanism does not functionsufficiently and the growth of necks is hindered and, as a result, thefilter obtained has had a low strength.

Incidentally, in DPF, it is important to reduce, as much as possible,the pressure loss which has a large influence on the output of anengine. To achieve this task, it is required to allow the DPF to have ahigher porosity, that is, to use, as the porous material constitutingthe DPF, a material of higher porosity. Also, for DPF catalystregeneration, it is required to suppress the pressure loss as much aspossible, by using higher porosity, specifically, a porosity of 50% ormore, particularly about 70%.

For obtaining a honeycomb structure having a higher porosity, there is aconventional method of adding a pore former such as starch, foamed resinor the like to a mixed raw material for a porous material (containingsilicon carbide particles, etc.) constituting the honeycomb structure tobe produced and, during firing of a formed material obtained, burningand blowing off the pore former. The amount of the pore former addedneed be large in order to achieve a porosity of certain level or higher,for example, a porosity of 60% or more; however, addition of a poreformer of organic compound type in a large amount results in generationof a large amount of a gas (e.g. a volatile organic substance and carbondioxide) during degreasing (calcination), as well as in generation oflarge combustion heat. The calcinated material (material afterdegreasing) or fired material obtained under such conditions has, insome cases, defects such as cracks, tears, cuts, large pores caused byagglomeration of organic pore former, and the like, that is, defectswhich do not allow proper functioning of filter and cause leakage offluid.

Patent Literature 1: JP-A-6-182228

The present invention has been made in view of the above-mentionedproblems of prior art and aims at providing a silicon carbide-basedporous material which is high in porosity and strength and superior inoxidation resistance and thermal shock resistance and, when used as afilter, is very low in risk of having defects such as cuts (they causeleakage of fluid) and the like as well as in pressure loss; a honeycombstructure constituted thereby; and a method for producing a siliconcarbide-based porous material having the above-mentioned properties.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a siliconcarbide-based porous material characterized by comprising siliconcarbide particles as an aggregate, metallic silicon and an oxide phasecontaining Si, Al and an alkaline earth metal.

In the present invention, it is preferred that the oxide phase is onsurfaces of and/or at circumferences of the silicon carbide particlesand/or the metallic silicon.

In the present invention, it is preferred that the oxide phase containsSiO₂, Al₂O₃ and an alkaline earth metal and the content of the alkalineearth metal relative to the oxide phase total is 9 to 50% by mass interms of the content of a monoxide of the alkaline earth metal.

In the present invention, it is preferred that the alkaline earth metalis at least one kind selected from the group consisting of Mg, Ca, Srand Ba.

In the present invention, it is preferred that the silicon carbide-basedporous material has a porosity of 50 to 80% and an average pore diameterof 10 to 50 μm; and it is also preferred that, when the average porediameter is expressed as 10^(x) μm and the distribution of the porediameters is expressed on volume basis, the total volume of pores(particular pores) whose diameters are in a range of 10^(x±0.25) μm, is80% or more of the volume of total pores.

In the present invention, it is preferred that the oxide phase covers atleast part of the surfaces of the silicon carbide particles and/or themetallic silicon.

In the present invention, it is preferred that at least part of theoxide phase is constituted by at least one kind selected from the groupconsisting of cordierite, anorthite, Sr feldspar (SrAl₂Si₂O₈) andcelsian (BaAl₂Si₂O₈).

According to the present invention, there is also provided a honeycombstructure characterized by being constituted by any of theabove-mentioned silicon carbide-based porous materials.

According to the present invention, there is also provided a method forproducing a silicon carbide-based porous material, characterized in thatit comprises adding, to a mixed raw material containing silicon carbideparticles and metallic silicon, inorganic microballoons containing Siand Al and a compound containing an alkaline earth metal, forming theresulting mixture into a formed material of intended shape, andcalcinating and firing the formed material to melt the inorganicmicroballoons to obtain a porous material of porous structure wherein anoxide phase containing Si, Al and an alkaline earth metal is on surfacesof and/or at circumferences of the silicon carbide particles and/or themetallic silicon.

In the present invention, it is preferred that the alkaline earth metalin the compound containing an alkaline earth metal is at least oneselected from the group consisting of Mg, Ca, Sr, and Ba.

In the present invention, it is preferred that the compound containingan alkaline earth metal is a compound which, when subjected to oxidationor decomposition, becomes at least one kind of alkaline earth metalmonoxide selected from the group consisting of MgO, CaO, SrO and BaO.

In the present invention, it is preferred that the inorganicmicroballoons are added in an amount of 5 to 30 parts by mass per 100parts by mass of the total of the silicon carbide particles and themetallic silicon.

In the present invention, it is preferred that the compound containingan alkaline earth metal is added, per 100 parts by mass of the inorganicmicroballoons added, by 10 to 100 parts by mass in terms of the amountof a monoxide of the alkaline earth metal including the alkaline earthmetal contained beforehand in the microballoons. It is further preferredthat the compound containing an alkaline earth metal is added, per 100parts by mass of the inorganic microballoons added, by 10 to 25 parts bymass in terms of the amount of MgO when the alkaline earth metalcontained in the compound containing an alkaline earth metal is Mg, by14 to 35 parts by mass in terms of the amount of CaO when the alkalineearth metal is Ca, by 26 to 64 parts by mass in terms of the amount ofSrO when the alkaline earth metal is Sr, or by 38 to 95 parts by mass interms of the amount of BaO when the alkaline earth metal is Ba.

In the present invention, it is preferred that the total content of theSi source and Al source contained in the inorganic microballoons,relative to the inorganic microballoons residual portion which is theinorganic microballoons total portion minus the alkaline earth metal(expressed as a monoxide thereof) contained beforehand in the totalportion, is 90% by mass or more when the Si source is expressed as SiO₂and the Al source is expressed as Al₂O₃, and the content of the Alsource contained in the inorganic microbaloons, relative to theinorganic microballoons residual portion which is the inorganicmicroballoons total portion minus the alkaline earth metal (expressed asa monoxide thereof) contained beforehand in the total portion, is 20 to55% by mass when the Al source is expressed as Al₂O₃.

In the present invention, it is preferred that the intended shape of theformed material is a honeycomb shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph showing the micro-structure of thesilicon carbide-based porous material of Example 1.

FIG. 2 is an electron micrograph showing the micro-structure of thesilicon carbide-based porous material of Example 2.

FIG. 3 is an electron micrograph showing the micro-structure of thesilicon carbide-based porous material of Example 3.

FIG. 4 is an electron micrograph showing the micro-structure of thesilicon carbide-based porous material of Example 9.

FIG. 5 is an electron micrograph showing the micro-structure of thesilicon carbide-based porous material of Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The mode for carrying out the present invention is described below.However, the present invention is not restricted to the following modeand it should be construed that design change, improvement, etc. can bemade appropriately based on the ordinary knowledge possessed by thoseskilled in the art as long as there is no deviation from the scope ofthe present invention.

The silicon carbide-based porous material of the present invention ischaracterized by comprising silicon carbide particles as an aggregate,metallic silicon and an oxide phase containing Si, Al and an alkalineearth metal. It is described in detail below.

The silicon carbide-based porous material of the present inventioncomprises silicon carbide particles as an aggregate and metallicsilicon. Therefore, in its production, firing can be conducted at arelatively low temperature; the production cost is low; and animprovement in product yield is obtained. Further, the present porousmaterial shows a high thermal conductivity because metallic silicon isused for bonding of silicon carbide particles which are refractoryparticles; and, when it is used, for example, as a DPF and when theparticulates deposited on the filter are burnt for filter regeneration,there hardly occurs such local temperature rise as to damage the filter.

Further, it comprises an oxide phase containing Si, Al and an alkalineearth metal; therefore, even when it is exposed to a low-oxygenatmosphere and a high temperature (both experienced when it is used as aDPF), oxidation and decomposition of silicon carbide and metallicsilicon are suppressed. Thus, the silicon carbide-based porous materialof the present invention is superior in oxidation resistance and thermalshock resistance and shows an effect that there is hardly the damage offilter caused by the heat generation due to, for example, the oxidationof silicon carbide and metallic silicon taking place during filterregeneration. It is preferred that the oxide phase is on surfaces of orat circumferences of the silicon carbide particles and/or the metallicsilicon because a higher effect is expected. In the present invention,it is more preferred in order to obtain improved oxidation resistanceand thermal shock resistance that the oxide phase covers at least partof the surfaces of the silicon carbide particles and/or the metallicsilicon. It is particularly preferred in order to obtain an improvedstrength that the oxide phase is filled in the pores of 10 μm or less tocomplement the necking between silicon carbide particles.

It is further preferred that the oxide phase contains SiO₂, Al₂O₃ and analkaline earth metal and that the content of the alkaline earth metal,specifically at least one kind selected from the group consisting of Mg,Ca, Sr and Ba, relative to the oxide phase total is 9 to 50% by mass interms of the content of a monoxide of the alkaline earth metal.

When the content of the alkaline earth metal relative to the oxide phasetotal is less than 9% by mass in terms of the content of a monoxide ofthe alkaline earth metal, a higher firing temperature is needed and,therefore, such a content is not preferred. When the content is morethan 50% by mass, the oxide phase has a too low a eutectic point, thefired material shrinks, and the porosity thereof decreases; therefore,such a content is not preferred. In order to form, by conducting firingunder given conditions, an oxide phase (feldspar) effective for higherthermal shock resistance, oxidation resistance and strength, such ascordierite, anorthite, Sr feldspar (SrAl₂Si₂O₈) or celsian (BaAl₂Si₂O₈),the content of the alkaline earth metal relative to the oxide phasetotal is particularly preferred to be 9 to 20% by mass as MgO, 12 to 26%by mass as CaO, 20 to 39% by mass as SrO or 28 to 49% by mass as BaO,according to the formula weights of the individual feldspars.Incidentally, specific conditions, etc. of firing are described later.

In the present invention, “the content of the alkaline earth metalrelative to the oxide phase total” refers to a value obtained byconducting EDS to an oxide phase (containing an alkaline earth metal)present in the polished surface of a silicon carbide-based porousmaterial and measuring the resulting characteristic X-ray of thealkaline earth metal, or a value obtained by determining the C, O, Si,Al and alkaline earth metal (Mg, Ca, Sr or Ba) of a siliconcarbide-based porous material using an elemental analysis apparatus(e.g. Simultaneous X-ray Spectrometer System) and then subtractingtherefrom the portions belonging to the silicon carbide particles andthe metallic silicon to obtain the remainder as the content inside theoxide phase. For example, there is calculated the amount of oxygen whenthe amount of Al obtained is expressed as the amount of Al₂O₃ and theamount of alkaline earth metal obtained is expressed as the amount ofits monoxide; this oxygen amount is subtracted from the total oxygenamount obtained; the remaining oxygen amount can be taken as the amountof SiO₂. Incidentally, the cordierite, anorthite, Sr feldspar(SrAl₂Si₂O₈) or celsian (BaAl₂Si₂O₈) constituting the oxide phase can beconfirmed by measurement of powder X-ray diffraction pattern andidentification. Examples of the measurement method, etc. have been shownabove; however, the measurement method for the content of alkaline earthmetal relative to oxide phase total and the method for identification ofcordierite, anorthite, Sr feldspar (SrAl₂Si₂O₈) or celsian (BaAl₂Si₂O₈)are not restricted to the above measurement and identification methods.

The silicon carbide-based porous material of the present invention ispreferred to have a porosity of 50 to 80% and further an average porediameter of 10 to 50 μm. A porosity of less than 50% or an average porediameter of less than 10 μm is not preferred because, when such asilicon carbide-based porous material is used in a DPF, particularly, aDPF for catalyst regeneration (wherein a catalyst component is coatedinside the pores at a later stage) or the like, the siliconcarbide-based porous material is unable to effectively carry thecatalyst component thereon. A porosity of more than 80% or an averagepore diameter of more than 50 μm is not preferred because it results ina very low strength and gives a DPF, a DPF for catalyst regeneration, orthe like, of insufficient durability. For a high porosity, a lowpressure loss and a high strength, the silicon carbide-based porousmaterial of the present invention is more preferred to have a porosityof 50 to 75% and an average pore diameter of 20 to 50 μm and isparticularly preferred to have a porosity of 55 to 70% and an averagepore diameter of 25 to 50 μm.

In the silicon carbide-based porous material of the present invention,in order to make low the leakage caused by large pores and effectivelylow the pressure loss, it is preferred that, when the average porediameter is expressed as 10^(x) μm and the distribution of the porediameters is expressed on volume basis, the total volume of pores(particular pores) whose diameters are in a range of 10^(x±0.25) μm, is80% or more of the volume of total pores. A total volume of particularpores, of 80% or less of the volume of total pores is not preferredbecause the number of pores having diameters much smaller or much largerthan the average pore diameter is large, small pores are unable toexhibit effective reduction in pressure loss, and large pores causesleakage of fluid and effective trapping of particulates, etc. is notmade. Incidentally, the average pore diameter (10^(x) μm) as a basepoint is appropriately selected in a DPF, particularly, a DPF forcatalyst regeneration (wherein a catalyst component is coated inside thepores at a later stage) or the like, depending upon the use conditions,the kind and amount of catalyst component carried, etc.

The honeycomb structure according to the present invention ischaracterized by being constituted by any of the above-mentioned siliconcarbide-based porous materials. This honeycomb structure is superior inoxidation resistance, acid resistance, resistance to reactivity withparticulates and thermal shock resistance, reflecting the properties ofthe silicon carbide-based porous material which constitutes thehoneycomb structure. Further, the present honeycomb structure can beused as a DPF, a DPF for catalyst regeneration, a catalyst carrier orthe like, under high SV (space velocity) conditions.

Description is made next on the method for producing the siliconcarbide-based porous material of the present invention. In producing thesilicon carbide-based porous material of the present invention, first,there is prepared a mixed raw material containing silicon carbideparticles and metallic silicon. Incidentally, the silicon carbideparticles and the metallic silicon contain, in some cases, a very smallamount of impurities such as Fe, Al, Ca and the like; however, they maybe used per se or after purification by chemical treatment such asreagent washing. To the prepared mixed raw material are added inorganicmicroballoons containing Si and Al and a compound containing an alkalineearth metal. Then, a forming aid such as organic binder or the like isadded as necessary, followed by mixing and kneading to obtain a clay forforming.

The inorganic microballoons function as a pore former when added to themixed raw material. As compared with conventional organic pore formerssuch as starch, foamed resin and the like, the inorganic microballoonshave a low specific gravity and an appropriate strength; therefore, theyhardly crumble during mixing and kneading and are easy to handle. Thepore former used may be totally the inorganic microballoons, or theinorganic microballoons may be used in combination with an organic poreformer. The clay obtained is formed into a formed material of intendedshape such as honeycomb shape or the like; the formed material iscalcinated to remove the organic binder contained therein (degreasing)and then fired; thereby, a silicon carbide-based porous material can beobtained.

In the present invention, a compound containing an alkaline earth metalis added simultaneously to the mixed raw material. The compoundcontaining an alkaline earth metal refers to such a compound that thealkaline earth metal contained therein is at least one kind selectedfrom the group consisting of Mg, Ca, Sr and Ba, and specifically is acompound such as strontium carbonate (SrCO₃), calcium acetate(Ca(CH₃COO)₂) or the like, which, in calcination or firing, is oxidizedor decomposed to become at least one kind of alkaline earth metalmonoxide selected from the group consisting of MgO, CaO, SrO and BaO. Byadding this compound, the inorganic microballoons melt in firing andform a porous structure of high porosity and also form an oxide phasecontaining Si, Al and an alkaline earth metal, on surfaces of and/or atcircumferences of the silicon carbide-based particles and/or themetallic silicon. This oxide phase is formed in an appropriate amount onsurfaces of and/or at circumferences of the silicon carbide-basedparticles and/or the metallic silicon; as a result, the resultingsilicon carbide-based porous material is improved in strength, oxidationresistance and thermal shock resistance.

The inorganic microballoons retain its shape after calcination. Theygenerate no gaseous component when fired. Therefore, the resultingsilicon carbide-based porous material hardly has defects such as crack,tear, cut and the like. They exhibit the same effects as mentioned aboveeven when used in combination with an organic pore former. In thepresent invention, it is preferred that the total content of the Sisource and Al source contained in the inorganic microballoons added,relative to the inorganic microballoons residual portion which is theinorganic microballoons total portion minus the alkaline earth metal(expressed as a monoxide thereof) contained beforehand in the totalportion, is 90% by mass or more when the Si source is expressed as SiO₂and the Al source is expressed as Al₂O₃ and that the content of the Alsource contained in the inorganic microballoons, relative to theinorganic microballoons residual portion which is the inorganicmicroballoons total portion minus the alkaline earth metal (expressed asa monoxide thereof) contained beforehand in the total portion, is 20 to55% by mass when the Al source is expressed as Al₂O₃, because suchinorganic microballoons have a lower eutectic point and can melt atlower temperatures. Preferred examples of such inorganic microballoonsinclude fly ash balloons (coal ash) which are generated as a waste inthermal power plants, etc., because effective utilization of waste ismade possible.

When the total content of the Si source and Al source contained in theinorganic microballoons added, relative to the inorganic microballoonsresidual portion which is the inorganic microballoons total portionminus the alkaline earth metal (expressed as a monoxide thereof)contained beforehand in the total portion, is less than 90% by mass whenthe Si source is expressed as SiO₂ and the Al source is expressed asAl₂O₃, the inorganic microballoons have too low a eutectic point, thefired material shrinks, and no sufficient pore forming takes place. Atotal content of less than 90% by mass is not preferred, either, becausethere is hardly formed an oxide phase such as cordierite, anorthite, Srfeldspar (SrAl₂Si₂O₃) or celsian (BaAl₂Si₂O₃), which is effective forimprovements in thermal shock resistance, oxidation resistance andstrength. There is no particular restriction as to the upper limit ofthe total content and a higher content is preferred theoretically. Whenthe content of the Al source contained in the inorganic microballoons,relative to the inorganic microballoons residual portion which is theinorganic microballoons total portion minus the alkaline earth metal(expressed as monoxide thereof) contained beforehand in the totalportion, is less than 20% by mass when the Al source is expressed asAl₂O₃, the SiO₂ in the inorganic microballoons is excessive and theinorganic microballoons are not melted; therefore, such an Al sourcecontent is not preferred. An Al source content of more than 55% by massis not preferred, either, because the SiO₂ is short and the inorganicmicroballoons are not melted. The content of the Al source is preferably25 to 50% by mass, particularly preferably 30 to 45% by mass in order toform an oxide phase which is effective for improvements in thermal shockresistance, oxidation resistance and strength.

In the present invention, “the total content of the Si source and Alsource contained in the inorganic microballoons when the Si source isexpressed as SiO₂ and the Al source is expressed as Al₂O₃” is a valuedetermined by Agglomerated mass plus absorptiometry and the EDTAtitration method conducted based on JIS M 8853 (methods for chemicalanalysis of refractory clay). Also, “the content of the Al sourcecontained in the inorganic microballoons when the Al source is expressedas Al₂O₃” is a value determined by the above methods.

The porosity and average pore diameter of the silicon carbide-basedporous material can be controlled by appropriately adjusting the size,addition amount, etc. of the inorganic microballoons. Incidentally,depending upon parameters such as the composition of the inorganicmicroballoons and the kind and addition amount of the compoundcontaining an alkaline earth metal, there vary the melting temperatureof the inorganic microballoons and the oxide phase formed. Therefore,from these parameters, there can be known beforehand the firingtemperature and the oxide phase of the silicon carbide-based porousmaterial obtained, and the material design and production design of theporous material can be made flexibly depending upon the use conditionsof the DPF of the like produced from the porous material.

In the present invention, the inorganic microballoons are preferred tohave a mean particle diameter of 100 μm or less because it enablesextrusion of a honeycomb having a partition wall thickness of 300 μm orless. This mean particle diameter is a value obtained by measurementusing a particle size distribution tester of laser scattering type. Theinorganic microballoons are also preferred to have a compressionstrength of 1 MPa or more as calculated with an assumption that eachinorganic microballoon is a solid sphere, because, at such a compressionstrength, crumbling takes place hardly during kneading. This compressionstrength is a value obtained by measurement using a minute compressiontester. The inorganic microballoons are also preferred to have a tappacking density of 0.4 g/cm³ or less and a shell thickness of 10 μm orless, particularly 5 μm or less. The shell thickness is a value obtainedby observation of the broken surface or polished surface using amicroscope. As a specific example of the inorganic microballoonssatisfying these conditions, there can be mentioned E-SPHERES SL-75 (aproduct of ENVIROSPHERES Co.); however, the inorganic microballoonsusable in the present invention are not restricted to such a specificexample.

In the present invention, the inorganic microballoons are added in anamount of preferably 5 to 30 parts by mass, more preferably 10 to 30parts by mass per 100 parts by mass of the total of the silicon carbideparticles and the metallic silicon. When the amount of the inorganicmicroballoons added is less than 5 parts by mass, no pore forming effectis exhibited; when the amount is more than 30 parts by mass, the amountof the oxide phase formed is too large and, resultantly, the firedmaterial shrinks and there is substantially no pore forming effect;therefore, such amounts are not preferred. Incidentally, when a higherporosity is desired, the inorganic microballoons may be used incombination with an organic pore former.

In the present invention, it is preferred that the compound containingan alkaline earth metal is added, per 100 parts by mass of the inorganicmicroballoons added, by 10 to 100 parts by mass in terms of the amountof a monoxide of the alkaline earth metal including the alkaline earthmetal contained beforehand in the microballoons. In order to form, inparticular, an oxide phase (feldspar) effective for improvement ofthermal shock resistance, oxidation resistance and strength, such ascordierite, anorthite, Sr feldspar (SrAl₂Si₂O₈) or celsian (BaAl₂Si₂O₈),the compound containing an alkaline earth metal is added, per 100 partsby mass of the inorganic microballoons added, by 10 to 25 parts by massin terms of the amount of MgO when the alkaline earth metal contained inthe compound containing an alkaline earth metal is Mg, by 14 to 35 partsby mass in terms of the amount of CaO when the alkaline earth metal isCa, by 26 to 64 parts by mass in terms of the amount of SrO when thealkaline earth metal is Sr, and by 38 to 95 parts by mass in terms ofthe amount of BaO when the alkaline earth metal is Ba. When the additionamount of the compound containing an alkaline earth metal is less than10 parts by mass in terms of the amount of a monoxide of the alkalineearth metal, the inorganic microballoons are difficult to melt; when theaddition amount is more than 100 parts by mass, excessive inorganicmicroballoons remain as such, the fired material shrinks, and nosufficient pore forming effect is obtained; therefore, such amounts arenot preferred.

Incidentally, in the present invention, the compound containing analkaline earth metal may contain only one kind of alkaline earth metalor two or more kinds of alkaline earth metals. Only one kind of compoundcontaining alkaline earth metal may be added, or two or more kinds ofcompounds may be added. When two or more kinds of compounds are added,the addition amounts of the individual compounds may be different fromeach other or may be equal to each other. By allowing both the compoundcontaining an alkaline earth metal and the inorganic microballoons to bepresent in the clay prepared, followed by firing, it is possible to meltthe inorganic microballoons to form a porous structure of high porosityand also an oxide phase. Further, by using inorganic microballoons whichare not blown off (not removed) when fired, the risk of formation oflarge pores is kept very low even if there arises agglomeration of poreformer (this becomes a problem with an organic pore former, etc.);consequently, the risk of fluid leakage is very low and theparticulates, etc. contained in a particle-containing fluid can betraped efficiently. Further, since the molten oxide phase fills minutepores, the necking between silicon carbide particles is complemented,achieving an increased strength. Furthermore, by forming, in the oxidephase, cordierite, anorthite, Sr feldspar (SrAl₂Si₂O₈), celsian(BaAl₂Si₂O₈) or the like, there can be obtained increases in thermalshock resistance, oxidation resistance and strength. Incidentally, asthe compound containing an alkaline earth metal, there is preferablyused a monoxide, carbonate or the like of an alkaline earth metal fromthe standpoints of efficient formation of oxide phase, availability,handleability, etc.

In the present invention, calcination is conducted preferably at atemperature lower than the temperature at which the metallic siliconmelts. Specifically explaining, calcination may be conducted at a giventemperature of about 150 to 700° C., or in a given temperature range ata small temperature elevation rate of 50° C./hr or less. Whencalcination is conducted at a given temperature, the given temperaturemay be one temperature level or a plurality of temperature levelsdepending upon the kind and amount of the organic binder used; and whena plurality of temperature levels are adopted, the time lengths of theindividual temperature levels may be the same or different. Whencalcination is conducted at a small temperature elevation rate, thesmall temperature elevation rate may be adopted only in one temperaturerange or a plurality of temperature ranges; and when the smalltemperature elevation rate is adopted in a plurality of temperatureranges, the temperature elevation rates in the individual temperatureranges may be the same or different.

In order to allow the silicon carbide-based porous material obtained tohave a porous structure wherein the refractory particles containedtherein are bonded by metallic silicon, the metallic silicon need bemelted during firing. Since the melting point of metallic silicon is1,410° C., the firing temperature is preferably 1,410° C. or more. Themost appropriate firing temperature is determined from themicro-structure and properties of the silicon carbide-based porousmaterial finally obtained. At a temperature higher than 1,600° C.,metallic silicon vaporizes, making bonding via metallic silicondifficult; therefore, the firing temperature is appropriately 1,410 to1,600° C., preferably 1,420 to 1,580° C.

The present invention is described more specifically below by way ofExamples. However, the present invention is in no way restricted tothese Examples.

EXAMPLES 1 TO 5 AND 7 TO 10

A SiC raw material powder having an average particle diameter of 47 μmand a Si powder having an average particle diameter of 5 μm werecompounded so as to give a composition of 80:20 in mass ratio. To 100parts by mass of the resulting powder were added fly ash balloons havingmass parts shown in Table 1, and there was (were) also added an alkalineearth metal monoxide(s) having mass parts shown in Table 1, relative to100 parts by mass of the fly ash balloons. Then, there were added 6parts by mass of methyl cellulose as an organic binder, 2.5 parts bymass of a surfactant and 24 parts by mass of water, followed by uniformmixing and kneading to obtain various clays for forming. Each clay wasformed by an extruder into a formed material of honeycomb shape havingan outer diameter of 45 mm, a length of 120 mm, a partition wallthickness of 0.43 mm and a cell density of 100 cells/in.² (16cells/cm²). Then, the formed material was calcinated for degreasing at500° C. for 5 hours and thereafter fired in an non-oxidizing atmosphereat 1,450° C. for 2 hours to produce various silicon carbide-based porousmaterials of honeycomb structure, of Examples 1 to 5 and 7 to 10.Incidentally, the electron micrographs showing the microstructures ofthe silicon carbide-based porous materials of Examples 1 to 3 and 9 areshown in FIGS. 1 to 4, respectively. Further, the oxide phase of eachporous material was identified by X-ray diffraction. The results areshown in Table 1.

EXAMPLE 6

A silicon carbide-based porous material of honeycomb structure ofExample 6 was produced by the same operation as in Examples 1 to 5 and 7to 10 except that a starch having an average particle diameter of 50 μmwas added as an organic pore former. Further, the oxide phase of theporous material was identified by X-ray diffraction. The results areshown in Table 1.

EXAMPLES 11 TO 13

Silicon carbide-based porous materials of honeycomb structure wereproduced by the same operation as in Examples 1 to 5 and 7 to 10 exceptthat the fly ash balloons were replaced by inorganic microballoonsdifferent in the content of total of Al₂O₃ and SiO₂ (Example 11) or byinorganic microballoons different in the content of Al₂O₃ (Examples 12and 13).

COMPARATIVE EXAMPLE 1

A silicon carbide-based porous material of honeycomb structure ofComparative Example 1 was produced by the same operation as in Examples1 to 5 and 7 to 10 except that there was used neither fly ash balloonsnor alkaline earth metal oxide. The electron micrograph showing themicrostructure of the silicon carbide-based porous material ofComparative Example 1 is shown in FIG. 5. Further, the oxide phase ofthe porous material was identified by X-ray diffraction. The results areshown in Table 1.

COMPARATIVE EXAMPLES 2 TO 3

A silicon carbide-based porous materials of honeycomb structure ofComparative Example 2 was produced by the same operation as in Example 6except that there was used neither fly ash balloons nor alkaline earthmetal oxide. Further, the oxide phase of the porous material wasidentified by X-ray diffraction. The results are shown in Table 1.

Evaluation of Physical Properties

Each of the silicon carbide-based porous materials produced was measuredfor the following physical properties. The results are shown in Table 1.

[Porosity]

Measured by the Archimedes method.

[Average Pore Diameter]

Measured using a mercury porosimeter.

[Control of Pore Diameters]

When the average pore diameter measured by the above mercury porosimeterwas expressed as 10^(x) μm and the distribution of the pore diameterswas expressed on volume basis, the ratio of the total volume of pores(particular pores) whose diameters were in a range of 10^(x±0.25) μm,relative to the volume of total pores was calculated and taken ascontrol (%) of pore diameters.

[Mass Increase in Oxidation Resistance Test]

An oxidation resistance test was conducted by a heat treatment in theair at 800° C. for 24 hours. The masses of each silicon carbide-basedporous material before and after the test were measured and the massincrease thereof was calculated according to the following expression(1).Mass increase (%)=[(mass after test)−(mass before test)]/ (mass beforetest)×100   (1)[Strength]

Four-point bending strength at room temperature was measured accordingto the method specified by JIS R 1601.

[Permeability]

Measured using Permporometer (a product of PMI Co.). Incidentally,“permeability” is a value indicating the flowability of a fluid per unitarea of filter material.

[Trapping Efficiency]

There was used an engine capable of generating a given amount ofparticulates per unit time. A filter paper was provided at the outlet ofthe engine; the mass of particulates deposited on the filter paper whenno filter (silicon carbide-based porous material) was fitted, was takenas 100; therefrom was subtracted the mass of particulates deposited onthe filter paper when a filter (silicon carbide-based porous material)was fitted; the remainder obtained was taken as “trapping efficiency(%)”.

TABLE 1 Mass Fly ash balloons Amount of Alkaline increase Al₂O₃ +organic earth metal Av. in Si/ SiO₂ pore monoxide pore Control oxidation4-point Permea- SiC Amount (%) former Amount Poros- diam- of poreresistance bending bility Trapping (mass (mass [Al₂O₃ (mass (mass Oxideity eter diameters test strength (×10⁻¹² efficiency ratio) parts) alone]parts) Kind parts) phase (%) (μm) (%) (%) (Mpa) m²) (%) Ex. 1 20/80 1098 Not used CaO 22 Anorthite 53 25 92 0 22 5.9 93 [35] Ex. 2 20/80 10 98Not used MgO 17 Cordierite 52 24 90 0 24 5.3 96 [35] Ex. 3 20/80 20 98Not used SrO 50 SrAl₂Si₂O₈ 50 30 91 0 30 7.8   9.1 [35] Ex. 4 20/80 1098 Not used CaO 40 Calcium 51 25 90   0.3 17 — — [35] silicate Ex. 520/80 10 98 Not used MgO 33 Forsterite + 53 25 89   0.3 19 — — [35]spinel Ex. 6 20/80 20 98 10 SrO, 30 SrAl₂Si₂O₈ + 64 31 85 0 15 18   89[35] CaO (SrO), 9 anorthite (CaO) Ex. 7 20/80  3 98 Not used CaO 22Anorthite 47 15 — — — — — [35] Ex. 8 20/80 40 98 Not used CaO 22Anorthite 49 20 — — — — — [35] Ex. 9 20/80 10 98 Not used CaO  9 Mullite53 9 — — — — — [35] Ex. 10 20/80 10 98 Not used SrO 105  Strontium 48 20— — — — — [35] silicate + strontium oxide Ex. 11 20/80 10 89 Not usedCaO 22 — 46 21 — — — — — [35] Ex. 12 20/80 10 98 Not used CaO 22 — 53 9— — — — — [15] Ex. 13 20/80 10 98 Not used CaO 22 — 53 9 — — — — — [65]Comp. 20/80 Not — Not used Not Not Not used 44 12 —   1.3 10 0.4 — Ex. 1used used used Comp. 20/80 Not — 30 Not Not Not used 62 20 68 —   5.24.3 72 Ex. 2 used used used Comp. 20/80 Not — 60 Not Not Not used 82 53— —   1.1 — — Ex. 3 used used used

When fly ash balloons and a compound containing an alkaline earth metalare appropriately added to a mixed raw material, followed by firing andmelting (Examples 1 to 5), it was found that the resulting siliconcarbide-based porous material can have a higher porosity and a largeraverage pore diameter and, when the average pore diameter of the porousmaterial is expressed as 10^(x) μm and the distribution of the porediameters of the porous material is expressed on volume basis, thediameters of the pores occupying 80% or more of the volume of totalpores can be controlled in a range of 10^(x±0.25) μm. It was found thatas a consequence, the pressure loss of the porous material issignificantly reduced (the permeability is increased significantly) andits efficiency for trapping particulates is increased. It was also foundthat the oxide phase generated by melting of fly ash balloons gives ahigher strength and a higher oxidation resistance. In contrast, in theconventional method, since the porosity and pore diameters of porousmaterial are small, the permeability is low and, when the porousmaterial is used as a filter, a large pressure loss arises (ComparativeExample 1).

With respect to the addition amount of the fly ash balloons, it wasfound that, when the addition amount is too small (Example 7), no poreforming effect appears and a porosity of 50% or less is obtained and,when the addition amount is too large (Example 8), the amount of theoxide phase formed is too large and, therefore, the fired materialobtained shrinks and no sufficient pore forming effect is obtained.

With respect to the addition amount of the oxide, it was found that,when the addition amount is too small (Example 9), there is no meltingof fly ash balloons, resulting in a small average pore diameter and,when the addition amount is too large (Example 10), an excessive portionof the oxide remains, the fired material obtained shrinks, and nosufficient pore forming effect is obtained. Further, by controlling, atan appropriate level, the addition amount of the oxide relative to theaddition amount of the fly ash balloons, it was found that anorthite,cordierite or Sr feldspar (SrAl₂Si₂O₈) is formed as an oxide phase and ahigher oxidation resistance and a higher strength are obtained(comparison of Examples 1 to 3 with Examples 4 and 5).

With respect to the content of Al source and Si source in fly ashballoons, it was found that, when the total content of the A1₂O₃ andSiO₂ contained in the fly ash balloons relative to the fly ash balloonsresidual portion which is the fly ash balloons total portion minus thealkaline earth metal contained beforehand in the total portion, is lessthan 90% (Example 11), too low an eutectic point results, the firedmaterial obtained shrinks, and no sufficient pore forming effect isobtained. It was also found that, even when the above total content is90% or more but when the content of Al₂O₃ is too small (Example 12),SiO₂ becomes excessive and there is no melting of the fly ash balloonsand, when the content of Al₂O₃ is too large (Example 13), SiO₂ becomesinsufficient and there is no melting of the fly ash balloons, either inthis case.

In FIG. 1 (Example 1) and FIG. 2 (Example 2), the black portions arepores, the gray portions are silicon carbide particles, the light grayportions are metallic silicon, and the dark gray (darker than siliconcarbide particles) portions which are present on surfaces of and/or atcircumferences of the silicon carbide particles and/or the metallicsilicon, are an oxide phase. Meanwhile, in FIG. 3, there is nodifference from FIGS. 1 and 2 with respect to the pores (blackportions), the silicon carbide particles (gray portions) and themetallic silicon (light gray portions), but the oxide phase is whitebecause it contains Sr. Thus, the color of the oxide phase in reflectedelectron image differs greatly depending upon the kind and amount of thealkaline earth metal added. It was found that there is an improvement inoxidation resistance when an oxide phase covers surfaces of siliconcarbide particles and metallic silicon as shown in FIGS. 1 to 3. It wasfurther found that the oxide phase is filled even in the fine poreportions (the portions where there is no necking by metallic silicon),complements the necking between silicon carbide particles, and gives anincreased strength. Meanwhile, in FIG. 5 (Comparative Example 1), therecan be confirmed, as in FIGS. 1 to 3, pores (black portions), siliconcarbide particles (gray portions) and metallic silicon (light grayportions), but it was impossible to confirm any portion corresponding toan oxide portion. Incidentally, in FIG. 4 (Example 9), there are seen,as in FIGS. 1 and 2, pores (black portions), silicon carbide particles(gray portions), metallic silicon (light gray portions) and an oxidephase [dark gray (darker than silicon carbide particles) portions];moreover, there can be confirmed those fly ash balloons which did notmelt during firing and remained (circular portions) and such aphenomenon could not be confirmed in FIGS. 1 to 3. Thus, it was foundthat when the addition amount of the compound containing an alkalineearth metal is appropriate (FIGS. 1 to 3), the fly ash balloons addedcan be melted totally and there can be obtained a sufficiently largeaverage pore diameter and a low pressure loss.

With respect to the pore former, when an organic pore former alone isused (Comparative Example 2), increases in both porosity and averagepore diameter are made possible; however, it gives a reduced strengthand further makes impossible the sufficient control of pore diametersdistribution, resulting in a significant reduction in particles-trappingefficiency. Particularly when the porosity obtained is 80% or more andthe average pore diameter obtained is 50 μm or more (Comparative Example3), there is a significant reduction in strength and no requiredstrength is obtained when the porous material obtained is used as afilter. In contrast, when the inorganic microballoons of the presentinvention and an organic pore former are used in combination (Example6), it was found that the porosity and average pore diameter obtainedare both increased greatly, yet the strength is improved and thedistribution of pore diameters can be controlled sufficiently,therefore, the permeability is increased and yet a articles-trappingefficiency is assured.

INDUSTRIAL APPLICABILITY

As described above, the silicon carbide-based porous material of thepresent invention and the honeycomb structure constituted by the siliconcarbide-based porous material comprises silicon carbide particles,metallic silicon and a desired oxide phase; therefore, they are high inporosity and strength and superior in oxidation resistance and thermalshock resistance and, when used as a filter, are very low in risks ofhaving defects such as cuts (which cause leakage of fluid), large pores(formed by agglomeration of organic pore former) and the like and,therefore, are low in pressure loss and yet assures an efficiency ofparticles trapping.

The method for producing a silicon carbide-based porous materialaccording to the present invention can produce, in given steps and undergiven conditions, a silicon carbide-based porous material whichcomprises silicon carbide particles, metallic silicon and a desiredoxide phase and which is high in porosity and strength and superior inoxidation resistance and thermal shock resistance, without generating,in the porous material, any defect such as cut or the like.

1. A silicon carbide-based porous material comprising silicon carbideaggregate particles bonded by metallic silicon particles constitutingnecking portions between bonded silicon carbide aggregate particles, andan oxide phase present in and strengthening said necking portions andcontaining Si, Al and an alkaline earth metal.
 2. The siliconcarbide-based porous material according to claim 1, wherein the oxidephase is present on surfaces of and/or at circumferences of the siliconcarbide particles and/or the metallic silicon.
 3. The siliconcarbide-based porous material according to claim 1, wherein the alkalineearth metal is at least one selected from the group consisting of Mg,Ca, Sr and Ba.
 4. The silicon carbide-based porous material according toclaim 1, which has a porosity of 50 to 80%.
 5. The silicon carbide-basedporous material according to claim 4, wherein when the average porediameter is expressed as 10^(x) μm and the distribution of the porediameters is expressed on volume basis, the total volume of pores(particular pores) whose diameter is in a range of 10^(x±−0.25) μm, is80% or more of the volume of total pores, wherein x is a numbercorresponding to an average pore diameter of 10 to 50 μm.
 6. The siliconcarbide-based porous material according to claim 1, wherein the oxidephase covers at least part of the surfaces of the silicon carbideparticles and/or the metallic silicon.
 7. The silicon carbide-basedporous material according to claim 1, wherein at least part of the oxidephase comprises at least one material selected from the group consistingof cordierite, anorthite, Sr feldspar (SrAl₂Si₂O₈) and celsian(BaAl₂Si₂O₈).
 8. A honeycomb structure comprising a siliconcarbide-based porous material according to claim
 1. 9. The siliconcarbide-based material according to claim 1, wherein the oxide phasecontains SiO₂, Al₂O₃ and an alkaline earth metal and the content of thealkaline earth metal relative to the oxide phase total is 9 to 50% bymass in terms of the content of a monoxide of the alkaline earth metal.10. The silicon carbide-based material according to claim 1, having anaverage pore diameter of 10 to 50 μm.
 11. The silicon carbide-basedmaterial according to claim 1, wherein the total volume of pores whosediameters are in a range of 10 to 50 μm is 80% or more of the total porevolume.